Magnetic recording media (disks) for use in magnetic recording-reproduction devices such as hard disk drives are prepared generally by forming a layer containing at least Ni and P over a nonmagnetic substrate, such as a glass substrate, to obtain a base plate and successively forming over the base plate a ground layer, magnetic layer, protective film and lubricating film.
Improved recording densities are required of magnetic recording media. To give an improved recording density, there is a need to reduce the distance between the head and the disk (head flying or levitation height). It is practice in recent years to hold the head almost in contact with the disk for recording or playback.
When signals are recorded or reproduced in such a quasi-contact state, the disk comes into contact with the head while rotating at a high speed, giving an increased load to the lubricating film of the disk. Consequently the lubricating film or protective film wears away, producing a deposit on the head, permitting the deposit to push up the head to a higher flying level or causing the head to levitate with impaired stability to render the head no longer operable properly for recording or reproduction. It is also likely that the lubricant forming the lubricating film will decompose to exhibit lower lubricity.
The reduction in the head levitation level further results in marked wear upon CSS (Contact, Start and Stop) and is likely to produce increased friction between the head and the disk or to cause damage to the head or the disk.
PFPE (perfluoropolyether) lubricants are known as typical lubricants for magnetic recording media. Examples of such PFPE lubricants include ZDOL (formula #101) and AM3001 (formula #102) commercially available from AUSIMONT in Italy, and DEMNUM SP3 (formula #103) and DEMNUM SA3 (formula #104) commercially available from Daikin Kogyo Co. Ltd. in Japan. These examples of lubricants are all brand names. EQU HOCH.sub.2 --{(CF.sub.2 (OC.sub.2 F.sub.4).sub.m --(OCF.sub.2).sub.n --OCF.sub.2 }--CH.sub.2 OH #101 ##STR1## EQU F--(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.p --CF.sub.2 CF.sub.2 --CH.sub.2 OH#104 ##STR2##
However, these lubricants fail to exhibit satisfactory lubricity during use while the head is in quasi-contact with the disk, and encounter difficulty in giving sufficient resistance to friction and abrasion.
Cyclotriphosphazine (brand name: X-1P, commercially available from Dow Chemical Co.) is used as a lubricant capable of giving high lubricity to the disk in quasi-contact with the head and greatly diminishing the abrasion of the disk and the head. X-1P is an organic compound having benzene rings and a phosphazine ring as represented by the formula #105 given above.
X-1P nevertheless has a molecular weight of about 1000 which is smaller than the conventional PFPE lubricants, and is inferior to the PFPE lubricants in adhesion to the carbon protective film, so that when singly used as a lubricant, X-1P not only spontaneously evaporates but also flows circumferentially off the disk if the disk is rotated for a long time, consequently gradually decreasing in quantity as applied to the disk and making it difficult to assure sufficient lubricity over a prolonged period of time.
Accordingly, it is attempted to admix a very small amount of X-1P with the conventional PFPE lubricant to prepare a lubricant composition for use in the form of a lubricating film. It has been found that the mixture of the PFPE lubricant and X-1P materially lessens the problem of friction between the head and the disk and abrasion thereof involved in the quasi-contact, serving to improve the performance of the disk.
The lubricant composition in the form of a mixture of PFPE lubricant and X-1P exhibits excellent lubricity when the head is in quasi-contact with the disk as stated above, whereas several days or several weeks after the composition is applied in the form of a lubricating film to the disk, the disk develops numerous small spots 12, about 1 .mu.m in diameter and hundreds of nanometers in height, over the surface 10 as shown in FIGS. 1 and 2 to cloud the disk surface. This phenomenon is called "haze", which is produced by X-1P applied onto the disk upon coagulating into small spots 12. It has been found that the haze occurs more frequently in an environment of higher humidity or temperature.
If the spots 12 of the haze become higher than the head flight level, the head collides with the spots during travel, permitting the lubricant composition to adhere to the surface of the head, or the haze spots will impede the normal travel of the head to result in seriously impaired recording or reproduction ability. There is also the likelihood that the head will be attracted to the disk to hamper the rotation of the disk.
Although the cause of haze still remains to be fully clarified, the present applicant has established that the haze is a phenomenon wherein the PFPE lubricant and X-1P are immiscible and separate each in the original state, forming a coagulation in the form of a thin film, and that the compatibility of the PFPE lubricant and X-1P is governed by the functional group of the PFPE lubricant.
However, AM type lubricants (such as AM3001), even in the original state, are compatible with X-1P, should therefore be less susceptible to hazing than ZDOL, but nevertheless eventually produce haze upon lapse of a long period of time when made into a lubricating film. We have found that the functional group of the AM type lubricant on the disk hydrolyzes mainly to hydroxyl at the portion of the benzyl position as represented by the following formulae, consequently rendering the lubricant no longer compatible with X-1P to result in haze.
Hydrolysis of functional group of AM type lubricant is shown in below: ##STR3##
FIG. 3 shows the decomposition of the functional group --C.sub.8 H.sub.7 O.sub.2 of AM3001 on the disk to --CF.sub.2 CH.sub.2 OH as determined by TOF-SIMS. With reference to FIG. 3, the proportion of the functional group --C.sub.8 H.sub.7 O.sub.2 decreases with time to increase the proportion of the functional group --CF.sub.2 CH.sub.2 OH. In FIG. 3, the initial value for the functional group --C.sub.8 H.sub.7 O.sub.2 at time 0 is taken as 10 as a standard, and the initial value for the functional group --CF.sub.2 CH.sub.2 OH at time 672 is standardized as 10.
Accordingly, we have developed organic compounds capable of retaining for a prolonged period of time a structure compatible with lubricating organic compounds such as X-1P containing a benzene ring and phosphazine ring by altering the structure of the terminal functional group of lubricants which is responsible for the hydrolysis described, whereby the present invention has been accomplished.
An object of the present invention is to provide novel organic compounds, and more particularly those having excellent lubricity, compatible with lubricating organic compounds having a benzene ring and a phosphazine ring and retaining stability free of hydrolysis for a prolonged period of time.